Materials in which the piezoelectric effect can be applied are being increasingly used in the production of sensors. This means that these materials deform when an electric voltage is applied. An important material parameter for the piezoelectric effect is the piezoelectric charge constant d. It describes the functional relationship between the applied field strength and the expansion of a piezoelectrically active material generated therewith. Thus, the piezoelectric charge constant d33 describes, for example, the expansion or compression of the piezoelectric material parallel to the applied electric field.
Films containing aluminum nitride (AlN) are known for their piezoelectric properties. Only a few processes with specific process parameters, which only allow a low tolerance range, are suitable, however, for the deposition of such films containing AlN in order to achieve good piezoelectric properties. A special crystal orientation of the AlN crystals, the so-called c-axis orientation, is necessary during the crystal growth of the AlN film if an AlN film with a piezoelectric charge constant d33 that is greater than zero is to be deposited.
It is known that the piezoelectrically active films containing AlN can be deposited by means of magnetron sputtering. In [J. X. Zhang, Y. Z. Chen, H. Cheng, A. Uddin, Shu Yuan, K. Pita, T. G. Andersson, Interface Study of AlN Grown on Si Substrates by Radiofrequency Magnetron Reactive Sputtering, Thin Solid Films 471, 2005, p. 336-341] it is proposed to deposit AlN films with c-axis orientation by means of reactive RF sputtering. The disadvantage of RF sputtering is that only low deposition rates of about 10 nm/min can be achieved during the deposition of piezoelectric AlN films.
In US 2009/0246385 A1 it is shown that piezoelectric AlN films can also be deposited by means of reactive pulse magnetron sputtering (20 kHz to 200 kHz) in addition to reactive RF sputtering. Sputtering methods with a DC power supply have considerable deficits with reference to process stability due to parasitic arc discharges.
Even if possibilities for the deposition of piezoelectric AlN films have been demonstrated with the previously described methods, they are nevertheless restricted in that mechanical stress within the deposited layers that can damage the film also occurs with these methods as the thickness of the film increases. Thus, in [Valery V. Felmetsger, Pavel N. Laptev, Roger J. Graham, Deposition of Ultrathin AlN Films for High Frequency Electroacoustic Devices, J. Vac. Sci. Technol. A 29(2), Feb. 14, 2011] it is disclosed that piezoelectric AlN films can only be deposited with sufficiently low mechanical stress up to a film thickness of about 2 μm by means of pulse magnetron sputtering at 40 kHz.